Edwin P. Plueddemann

Adhesion Through Silane Coupling Agents

ABSTRACT From a study of silane coupling agents in reinforced plastics, a general mechanism of adhesion to hydrophilic mineral surfaces has been devised. According to this theory, adhesion of polymers to dissimilar surfaces is described as a dynamic equilibrium of making and breaking of adhesion bonds between polymer segments and the surface through the agency of a low molecular weight material—usually water. A dynamic equilibrium at the interface allows relaxation of thermal stresses. Water resistance results from a favorable equilibrium toward bonding through polar groups in the polymer. Silanol groups generally give optimum bonding to hydrophilic mineral surfaces. Such a dynamic mechanism of adhesion not only explains many complex adhesion problems of plastics to mineral surfaces, but also is compatible with the adhesion of ice, barnacles and tooth plaque to surfaces in an aqueous environment and with the requirements for rubber reinforcement by finely divided particulate fillers.

Methods for improving the performance of silane coupling agents

The use of silane coupling agents in mineral- and glass-reinforced composites is well known. They impart improved initial mechanical properties, but, more importantly, they cause mechanical properties to be retained during the use of the composite. The main cause of loss of mechanical properties is attack of water at the interface. Recent research has focused on imparting more durable bonding of the silane coupling agent to both the polymer and the reinforcement. Improved silane coupling agent systems have been developed by utilizing several techniques: blends of hydrophobic silanes with hydrophilic silanes to give greater hydrophobic character; use of 1,2-bis-(trimethoxysilyl)ethane as an additive to give increased siloxane crosslinking; use of more thermally stable silanes such as phenyltrimethoxysilane and N-[2-(vinylbenzylamino)-ethyl]-3-aminopropyltrimethoxysilane to give increased thermal stability; and the use of a carboxy-functional silane with a carboxy-functional polymer and zinc salt to give io...

Reminiscing on silane coupling agents

—When K. L. Mittal asked me to provide a historical account of the applications of silane coupling agents in adhesion, I decided to write in the form of a personal account of my last 45 years in this line of study. No attempt is made to make the history comprehensive, or to recognize the host of other researchers who have contributed to our understanding of adhesion across an interface of dissimilar materials. It has been an immensely interesting area of study with many practical applications in composites and bonded structures.

Adhesion mechanism of polyvinyl chloride to silane primed metal surfaces

The interface of partially hydrolyzed organosilane primer Dow Corning® Z-6020 [N-(2-aminoethyl-3-aminopropyl trimethoxy silane], which will be referred to in this paper as simply AEAPS, and polyvinyl chloride (plastisol) was examined by XPS (X-ray photoelectron spectroscopy) depth profile analysis. These studies and the adhesion strength measurements under dry and wet conditions showed that the net bond strength and its resistance to moisture are determined by two factors: the extent of crosslinking of the primer and the interdiffusion of the two phases. Best adhesion is achieved as a result of the optimal balance between the above two conflicting tendencies, i.e. when the primer crosslinks after it has diffused into the polymer.

Chemistry of Silane Coupling Agents

The “coupling” mechanism of organofunctional silanes depends on a stable link between the organofunctional group (Y) and hydrolyzable groups (X) in compounds of the structure X3SiRY. The organofunctional groups (Y) are chosen for reactivity or compatibility with the polymer while the hydrolyzable groups (X) are merely intermediates in formation of silanol groups for bonding to mineral surfaces.

Silane adhesion promoters in coatings

Abstract Adhesion of coatings through silane primers or additives involves a complex interplay of chemical and mechanical properties of the interphase region. Some general concepts are described for optimizing the performance of silane adhesion promoters. A few examples are given of recommended primer formulations and applications in specific systems. These should serve as guidelines for formulating primers and additives for commercial coatings.

Silane primers for epoxy adhesives

A rapid screening test is described for measuring the water resistance of epoxy bonds to glass and metals. Thin films of epoxy, cured on glass or metal coupons, were soaked in water until the films could be loosened with a razor blade. Penetration of water to the interface was almost immediate, magnifying the differences in times to failure between primed and unprimed surfaces. Comparative qualitative ratings are given for standard silane primers with typical epoxy formulations. Additional modifications of primers are suggested for optimizing the water resistance of the bonds. Test results show that proper formulation of silane adhesion promoters may provide very water-resistant bonds between epoxies and metals or glass.

Composites having ionomer bonds with silanes at the interface

—Silane coupling agents have been developed for bonding virtually all thermoset and thermoplastic polymers to glass and metal surfaces. Bonding to the polymer is generally through interdiffusion of oligomeric siloxanes at the interface with possible crosslinking to interpenetrating polymer networks in the interphase region. Such structures provide very stable, water-resistant bonds across the interfaces for composites prepared under low shear. Under high shear, such as in injection molding, the bonds that are initially formed are torn by mechanical and thermal stress. A new mechanism of bonding across the interface has been discovered in which acid-functional silanes on the substrate bond to acid groups in the polymer in the presence of metal ions through ionomer bonds. Ionomers are fluid under conditions of high temperature and high shear, but reform at room temperature to tough water-resistant structures.

Nature of Adhesion through Silane Coupling Agents

Most thermosetting polymers undergo some shrinkage during cure. This may range from 6 to 10% for polyesters to 3 to 4% for epoxies. Such shrinkage in the presence of fillers or reinforcements sets up stresses at the polymer-resin interface. Bush1 embedded strain gauge transducers in a polyester resin cured at 120°C and measured internal static pressures of 14 to 21 MPa (2000–3000 psi). Bulk mold shrinkage may be reduced to less than 1% by loading the polymer with particulate fillers, but interfacial stresses are merely transferred to the filler surfaces.

Silane Adhesion Promoters for Polymeric Coatings

Trace amounts of certain organofunctional silicon compounds are used as primers or additives to improve adhesion of organic coatings to mineral substrates by a “coupling” mechanism that combines organic and inorganic reactivity through a stable molecular bridge.